Relatively low-power (e.g., 10 dB above noise floor) signals encoding digital information can be admixed together with composite video signals without being readily evident in television pictures generated from those composite video signals if suitable restrictions on the digital signal format are observed, the inventors point out. Preferably, the digital signals are used to modulate a subcarrier of a suppressed carrier of the same frequency as the video carrier. Then, long runs of ZEROs or ONEs do not have an appreciable effect on the low-frequency content of the frames, so as to affect in any way synchronization and automatic gain control operation in a television receiver already in the field. In order not to interfere with the recovery of sound in the television receiver, the subcarrier and its sideband structure should not have appreciable energy in the vicinity of the frequency-modulated audio carrier. Most of the energy in the luminance component of a composite video signal is on average in a frequency band below about 2 MHz, which fact is taken advantage of in VHS video tape cassette recording, for example.
It is advantageous to use a suppressed, vestigial-sideband, amplitude-modulated (VSB AM) carrier of the same frequency as the VSB AM picture carrier, but in quadrature phasing therewith, on which to transmit the subcarrier modulated with digital data. This procedure permits the synchronous detection of that quadrature carrier to recover the composite video signal without substantial energy in the baseband extending up to the 0.75 MHz frequency, at which frequency the VSB AM video carrier begins the transition from being a double-sideband amplitude-modulated (DSB AM) carrier to being a single-sideband amplitude-modulated (SSB AM) carrier, and at lessened energy up to the 1.25 MHz frequency at which roll-off of the vestigial sideband is complete. This procedure also lessens the visibility of the sideband of the subcarrier modulated with digital data to the extent it occupies the band where the VSB AM carriers are DSB AM in character.
The subcarrier for the digital signals should be chosen so that it and its sideband structure are discriminated against by the conventional chroma bandpass filter. If one attempts to solve this problem in terms of a single-dimension frequency spectrum, rather than in terms of a plural-dimension frequency spectrum using comb-filtering, there are severe constraints on the spectrum available for the transmission of data. The lower-frequency sideband of the I-channel chrominance extends down at least 1.3 MHz in frequency from the 3.58 MHz subcarrier, reaching down to the frequencies where there is significant energy in the luminance signal; and the upper-frequency sidebands of the I-channel and Q-channel chrominance extend up 0.5 MHz in frequency from the 3.58 MHz subcarrier. The problem is more tractable when treated in terms of a two-dimensional frequency spectrum using line comb filtering.
Line-comb bandpass filtering is often used, either alone or in cascade with a conventional chroma bandpass filter, for separating chrominance sidebands from luminance. The line-comb bandpass filtering suppresses cross-luminance because of the pronounced line-to-line correlation of luminance signal. Transmitting the same digital signals in each consecutive pair of successive scan lines provides for perfect line-to-line correlation of the digital signal sidebands in corresponding pixels of the two scan lines in each such consecutive pair, providing that the digital signal is encoded in the sideband frequencies of a subcarrier that does not exhibit 180.degree. spatial phase shift from scan line to scan line. Then, line-comb bandpass filtering suppresses crosstalk of the digital signal sidebands into alternate scan lines of its mostly-chrominance-signal response.
More importantly, inasfar as the invention in certain of its aspects is concerned, line-comb lowpass filtering suppresses cross-chrominance in alternate scan lines of its response. In an aspect of the invention, this suppression of cross-chrominance in alternate scan lines of the line-comb lowpass filter response is preferably made nearly perfect by averaging color-difference signals in each consecutive pair of successive scan lines. Such averaging provides for perfect line-to-line anti-correlation of the orthogonal AM sidebands of the complex color subcarrier in corresponding pixels of the two scan lines in each such consecutive pair. Such averaging reduces the vertical spatial resolution of chroma in each frame, but this reduction of vertical resolution is not apparent at normal viewing distances. The line pairing in chroma averaging is interleaved in vertical spatial phasing from frame to frame, so the resolution loss is reduced for static portions of the televised image. Prior to such averaging, the vertical spatial resolution of chroma in each frame is the equal of the vertical spatial resolution of luma, while the horizontal spatial resolution of chroma in each frame is only one-sixth or one-seventh the horizontal spatial resolution of luma.
A subcarrier that is a multiple of scan line rate does not exhibit 180.degree. spatial phase shift from scan line to scan line. A subcarrier that is an odd multiple of half scan line frequency, but has its phase shifted 180.degree. between the end of each scan line and the beginning of the next, does not exhibit 180.degree. spatial phase shift from scan line to scan line. The sidebands of either of these subcarriers do not exhibit 180.degree. spatial phase shift between two scan lines in which the modulating signal is repeated. Therefore, with either of these modulated subcarriers, the separation of the modulated subcarrier and chrominance from each other using line-comb filtering is good. The choice of a subcarrier that is an odd multiple of half scan line frequency, but has its phase shifted 180.degree. between the end of each scan line and the beginning of the next, will be shown further on in this specification to be preferable with regard to spectral interleaving with the luminance signal.
When the subcarrier can be separated nearly perfectly from the chrominance sidebands by lowpass line-comb filtering, a frequency band becomes available for the transmission of digital data without substantial interference by components of the composite video signal. This band is below the 4.2 to 4.8 MHz band occupied by the frequency-modulated 4.5 MHz sound carrier and is above the band below about 2.5 MHz wherein most of the energy in the luminance component of a composite video signal is on average. Filtering procedures to separate the subcarrier modulated by the digital signals from the luminance component of a composite video signal have been considered by the inventors. One objective of such filtering is to reduce the luminance-signal interference with the transmission of digital data in this 2.5 to 4.2 MHz band. Another objective is to reduce the luminance-signal interference with the transmission of digital data at frequencies below 2.5 MHz, particularly those frequencies above the zero to 0.75 MHz band freed of interfering luminance signal components by using a quadrature-phase VSB AM carrier for the transmission of digital data.
Unlike the channel for chrominance signal, the channel for luminance signal does not have excessive spatial or temporal resolution for all possible signal conditions. Nevertheless, on average, the luminance signal does exhibit high degrees of correlation between corresponding picture elements (or "pixels") in consecutive frames and between pixels in the same locality within a frame. Causing the subcarrier modulated by the digital signals to exhibit high degrees of anti-correlation between corresponding picture elements (or "pixels") in consecutive frames will provide a basis for separating the digital signal sidebands from the luminance signal by bandpass frame-comb filtering. Bandpass frame-comb filtering also provides a basis for separating the digital signal sidebands from the luminance signal of the adjacent channel next higher in frequency, which can be, taken, advantage of when designing the intermediate-frequency (IF) amplifiers for the digital signal receiver.
The visibility of the signals encoding digital information in the luminance components of the color signals supplied to the television picture display apparatus is reduced by transmitting the same sequence of digital signals in each frame of a successive group of M consecutive frames, but with opposite spatial phasing of the subcarrier in scan lines occurring during odd-numbered frames from that in corresponding scan lines occurring during even-numbered frames, where:
(1) the scan lines in each frame are considered to be consecutively numbered in order of their occurrence beginning with one,
(2) the frames are considered to be consecutively numbered modulo-M in order of their occurrence,
(3) M=2.sup.N, and
(4) N is a positive integer equal to or greater than one.
In the interest of keeping data rate high and reducing the number of frames that have to be stored, N is preferably chosen to be one, with the same digital signals being transmitted in consecutive pairs of frames but modulating subcarrier in opposite spatial phasing in corresponding scan lines in the two frames of each pair. If the subcarrier for digital data is an odd multiple of half scan line frequency having its phase shifted 180.degree. between the end of each scan line and the beginning of the next, frame-to-frame reversal of subcarrier phase in corresponding scan lines of consecutive frames occurs automatically. If the subcarrier for digital data is an even multiple of half scan line frequency with phase continuity from one scan line to the next in each frame, frame-to-frame reversal of subcarrier phase in corresponding scan lines of consecutive frames requires a switch in subcarrier phase to be made at corresponding points in each consecutive frame.
The repetition of the digital information in similar spatial phasing in each successive pair of line scan intervals and the further repetition of the digital information in opposite spatial phasing in each successive pair of frames reduces the channel capacity for digital signal (presuming the absence of interfering composite video signal) four times, but significant advantages are obtained in return. There is a significant reduction in the amount of interference from the composite video signal, owing to cancellation of portions of that signal during the comb-filtering processes. In particular, chrominance information and static luminance information are almost completely removed from the frequency spectrum. At the same time the digital data combines in accordance with algebraic addition, while random noise combines in accordance with vector addition, to increase the energy of the digital data by 6 dB respective to random noise and by at least 3 dB respective to the luminance signal components that change from frame to frame. Furthermore, impulse noise that rings the video IF amplifier is much less likely to destroy the digital information in four scan lines than in one. These improvements in signal strength versus noise translate into a lessening of the expected error rate and in a reduced need for error-correcting coding; the reduced need for error-correcting coding lessens the overhead associated with such coding and gains back some portion of the channel capacity.
The repetition of the digital information in similar spatial phasing in each successive pair of line scan intervals and the further repetition of the digital information in opposite spatial phasing in each successive pair of frames places the digital information in portions of the spectrum known to television engineers as the Fukinuki "holes". See for example, T. Fukinuki et al., "Extended Definition TV Fully Compatible with Existing Standards", IEEE Transactions on Communications, Vol. COM-32, No. 8, August 1984, pages 948-953; and T. Fukinuki et al., "NTSC FULL COMPATIBLE EXTENDED DEFINITION TV PROTO MODEL AND MOTION ADAPTIVE PROCESSING", reprinted from IEEE Communications Society IEEE Global Telecommunications Conference, No. 4.6, Dec. 2-5, 1985, pages 113-117; the disclosures of which are incorporated hereinto by reference thereto. In practice, the use of the Fukinuki "holes" for the transmission of video information has not proven to be completely satisfactory; the spatial and temporal correlation/anti-correlation patterns of additional video information prevent the degree of randomness of signal that is necessary for its being completely hidden in a normal television picture received by TV receivers already in the field. Attempts have been made by extended-definition television (EDTV) system designers to randomize the scanning pattern of the additional video information, in order to avoid its appearing in phantom form in the normal television picture. The problem of phantom Fukinuki-hole information appearing in the normal television picture is less severe where the energy of the information is low, as is the case with the subcarrier modulated with digital information, than when the energy of the Fukinuki-hole information has to be substantially as high as normal composite video signal, as is the case with extended video information. Transmitting the digital data on a quadrature VSB AM carrier suppresses the portions of the Fukinuki phantoms in the horizontal spatial frequencies below about 1 MHz which would otherwise be present in the in-phase video detector response.
Digital information unrelated to video and transmitted at high symbol rates tends to be spatially random, and coding techniques can be employed to increase the degree of spatial randomness. While in each consecutive pair of scan lines the digital information is repeated, as presented on a TV viewing screen these scan lines are separated by an intervening scan line because of the spatial interleaving in each TV frame of the scan lines of its final field with the scan lines of its initial field. Supposing the digital information has been encoded so as to be substantially spatially and temporally random as mapped against the raster scanning of a television screen, if one chooses a suitable form of modulation of the subcarrier for that digital information, it should be less distinguishable from noise than raster-scanned video information. Single-sideband amplitude-modulation (SSB AM) with suppressed subcarrier is the simplest modulation scheme that achieves substantially the maximum data rate for a given bandwidth. On-off keying (OOK), which codes ZEROs and ONEs as absence of carrier and presence of carrier, can result in patchiness in the "noise" contributed by the signals encoding digital information, particularly for coding schemes that allow long runs of ZEROs or ONEs, and particularly when considering images one frame at a time. This could be a lack of randomness that would occasionally cause the Fukinuki-hole information appearing in the normal television picture to be a perceptible phantom.
The inventors point out that a "constant-power" modulation scheme, such as provided by various forms of phase shift keying (PSK), is preferable from the standpoint of a viewer of Fukinuki-hole phantoms assessing them as being characterless noise and disregarding their presence. It is generally known by communications engineers that PSK has better capability of rejecting broad-spectrum interfering or "jamming" signals than any other form of digital modulation. The composite video signal can be considered to be a broad-spectrum interfering or "jamming" signal, the inventors point out. Repeating the PSK so as to place its spectra into the Fukinuki holes permits portions of this jamming signal to be eliminated by comb filtering.
When there is appreciable frame-to-frame change in luminance signal component in those areas of the television pictures generated from the composite video signals modified to include digital signal content, bandpass frame-comb filtering in the digital signal receiver cannot completely remove the luminance signal accompanying the digital signal sidebands. This is so in portions of frames containing moving images and is so in a pair of frames when a cut between camera takes occurs therewithin. Where an area of remnant luminance signal extends over a significant number of pixels in the horizontal direction or vertical direction, there is a high likelihood of appreciable local spatial correlation amongst adjacent pixels in that direction and thus of continuity of luminance energy across symbol boundaries of the digital information as mapped to image space. When crossing a symbol boundary in the horizontal direction or vertical direction, going from one random symbol to another, digital information has a 50% chance of being a ONE or ZERO in the random symbol gone to. These facts manifest themselves in the frequency spectra of the PSK and of the remnant luma having little interaction with each other. Consequently, the synchronous detection of the PSK carrier in the horizontal direction during symbol recovery will strongly discriminate in favor of the PSK modulating signal and generally will strongly discriminate against the remnant luminance signal.
The frequency spectrum of components of the composite video signal descriptive of fast-slewing video signal transitions at edges transversal to scan lines can be made to be less interfering with the frequency spectrum of the subcarrier with keyed phase shift, the inventors point out, if the subcarrier is an odd multiple of half scan line frequency and has its phase shifted 180.degree. between the end of each scan line and the beginning of the next. The reduced interference between the frequency spectra will reduce the incidence of error associated with the synchronous detection of symbols. Most of the high-frequency energy in a luminance signal having edges transversal to scan lines is located at even multiples of half scan line frequency. While the digital codes used as keying signal are frequently chosen to have the same number of ZEROs and ONEs in order to suppress the subcarrier, so the subcarrier being located at an odd multiple of half scan line frequency would appear not to be particularly significant, choosing the symbol rate to be a multiple M times scan line frequency will cause a substantial portion of the energy in the sidebands of the subcarrier to be located close to odd multiples of half scan line frequency, providing that M=2N, where N is a positive integer as large as permitted by the video bandwidth available.
For example, choosing N=8 results in a SSB BPSK signal with 256 f.sub.H =4,027,971 Hz bandwidth, in which a symbol variation (n/256) times maximum symbol rate places a respective sideband at an odd multiple of half scan line frequency, where n is any positive number from 1 to 256. Such an SSB BPSK signal can be formed as the upper sideband of a DSB BPSK carrier and separated by filtering for downconversion in frequency to a 6.5 f.sub.H =102,273 Hz subcarrier. The resulting SSB BPSK signal extends from 102,273 Hz to 4,130,244 Hz, so it does not interfere with the FM sound carrier offset from the video carriers so as to be in a 4.2-4.8 MHz band as referred to "zero" video carrier frequency. Since the SSB BPSK signal is an "upper" sideband, there is no appreciable energy between the half band frequency of 2,116,259 Hz and the uppermost frequency of 4,130,244 Hz to interfere with the chrominance sidebands of the color subcarrier. As compared to QPSK or MPSK, which are invariably DSB in nature, the inventors find "upper" SSB BPSK is better able to avoid putting appreciable energy into the chrominance sidebands of the color subcarrier, while utilizing as much of the baseband bandwidth of the composite video signal as possible. The reduction of energy in the portion of the spectrum in which chroma resides in the composite video signal substantially reduces the "chroma noise" perceived by a person viewing the screen of a TV receiver that separates chroma from luma by conventional bandpass filtering rather than by bandpass line-comb filtering, when the composite video signal with PSK subcarrier buried therein is being received.
The inclusion of error-correcting codes in the digital data is advisable. A principal reason for using such error-correcting codes is to be able to correct in the digital signal receiver those burst errors caused by impulse noise, but the error correction procedures correct as well those errors arising from remnants of luminance and chrominance signals that are left after comb filtering and are of such nature as to cause jamming interference with the PSK. Another principal reason for using such error-correcting codes is to be able to correct for inter-symbol error arising from signaling close to the channel bandwidth capabilities. Coding of the digital signal can also be done prior to its transmission so as to randomize the patterns in the Fukinuki phantoms when the data tends to be repetitive, and appropriate decoding will then be done in the digital receiver.
There is a substantial amount of synchronizing information available in the composite video signal that the PSK is buried in, so differential encoding in which ZEROs and ONEs are encoded by absence of or presence of phase shift at each predicted keying interval is not necessary. Direct encoding of ZEROs and ONEs as respective phases of the PSK subcarrier can be used, in which pairing of errors is less likely to occur. On the other hand, timing errors can arise during comb-filtering of redundant PSK subcarrier from the composite video signal and are better tolerated by PSK that uses differential encoding.